Because of its heat resistance, weather resistance, durability and electrical properties, silicone rubber has been widely employed in a variety of fields as building materials, electric and electronic parts, automotive parts, and business machine parts. Of these, the automotive parts of silicone rubber include oil seals, connector packings and plugs, O-rings, diaphragms, and distributor grommets. Especially for use as grommets in connectors and distributors where ease of assembly, tight seal when mounted, water-proofness, and insulation are of particular concerns, oil-bleeding silicone rubber was found effective because molded parts thereof allow oil to bleed on their surface. In fact, oil-bleeding silicone rubber has been in widespread use. The bleed oils used in the prior art are often silicone oils containing phenyl groups and having a viscosity of 100 to 500 centistokes.
Molded parts of such oil-bleeding silicone rubber can be produced by molding a silicone composition into a desired shape by conventional rubber molding techniques such as compression molding, transfer molding and injection molding, followed by vulcanization and curing in a conventional manner. Many molded parts are of complex shape and of small size. Also in most cases, several tens to several hundreds of parts are made in one molding operation. To increase productivity under the circumstances, it is requisite that molded parts can be readily removed from the mold.
For quick removal from the mold, automatic ejection relying on mechanical forces by ejectors or pneumatic pressure is often used. If a certain force is required for removal because of the close adhesion of a part to the mold, it is necessary to take a longer time than usual for the removal operation. In some cases, removal by mechanical forces is impossible and parts must be manually taken out. This is time consuming and accompanied by the risk of damaging the part or the mold or both. Especially for unmanned operation of an injection molding machine, ease of removal is important. In a single mold capable of simultaneously molding a plurality of parts in one operation, if any of the parts is left affixed in the mold, the machine must be shut down and that part must be manually taken out. This represents a substantial hindrance to productivity.
At the same time, several techniques of applying an abherent to the surface of a mold, coating the surface of a mold, and adding an abherent to a silicone rubber composition are used to reduce the resistance to separation. The oil-bleeding silicone rubber generally has a less resistance to separation because oil bleeds out to the mold surface upon vulcanization and curing. However, in the case of automatic removal of molded parts by means of an ejector, the ejector can slip on the part surface depending on the degree of oil bleeding or penetrates into a recess or aperture in the part, resulting in a failure of removal.
The oil-bleeding silicone rubber has another problem. After the completion of curing, some of bled oil is left on the mold surface. As the molding operation is repeated, such oil deposit contaminates the mold surface. If the molding step is further repeated without cleaning, stains will transfer to molded parts or the removal of parts from the mold is interfered, eventually resulting in such problems as inhibited removal due to adhesion of molded parts to the mold, damages on molded parts, detrimental influence on curing, and corrosion of the mold. To prevent such inconveniences, the mold must be periodically cleaned. In order to clean the mold which is used on-line, the mold must be detached from the molding machine, cooled down to room temperature, cleaned with chemical agents and solvents, mounted again in the molding machine, and allowed to stand until the mold temperature rises and stabilizes at the molding temperature. This also interferes with efficient production.
As mentioned above, the bleed oils commonly used in the prior art are silicone oils containing about 15 to 40 mol % of phenyl groups and having a viscosity of 100 to 500 centistokes. Those silicone oils which are likely to bleed out and facilitate removal of rubber parts from the mold, that is, low-viscosity, high phenyl content silicone oils cause more mold contamination. They do not satisfy both the requirements of smooth separation and least contamination.
JP-A 6-16938 discloses an oil-bleeding silicone rubber composition having blended therein a bleed oil containing specific amounts of hydroxyl and phenyl groups. JP-A 6-93186 discloses an oil-bleeding silicone rubber composition having blended therein two types of bleed oil. These compositions do not yet satisfy both the requirements of smooth separation and least contamination. These references indicate nowhere the characteristic component (C) of the present invention which is a bleed oil having methyl and phenyl groups attached to substantially all silicon atoms.